1. Field of the Invention
The present invention concerns a feedback compensator in an acoustic amplification system to compensate a feedback signal that ensues upon amplification of an input signal due to a feedback path from an amplified output signal to the input signal. The invention also is directed to a hearing aid device with such a feedback compensator, a method for compensating a feedback signal in an acoustic amplification system, and a method for compensating feedback in a hearing aid device.
2. Description of the Prior Art
In hearing aid devices, the problem commonly exists of an undesired acoustic feedback between a speaker (earphone) and a microphone. Such a feedback can cause whistling noises or other disturbances and thereby significantly diminish, or even reduce to zero, the usefulness of the hearing aid device for the user. Depending on the characteristics of the hearing aid device and the auditory situation, a feedback can ensue at different frequencies.
By means of adaptive feedback compensators, it is known to generate a compensation signal that is subtracted from the input signal before the amplification, and thus a frequency causing the feedback is reduced to an intensity that is below the stability limit. The generation of the compensation signal ensues along a feedback compensation path with an adaptive feedback compensation filter that is adjusted by means of an adaptation unit and reproduces the feedback path. A frequency-limiting filter in the feedback compensation path limits the frequency range in which the compensation signal is generated.
The expenditure for realizing such feedback compensators is significant, due to the generation of the feedback compensation path equivalent to the feedback. The generation of the feedback compensation signal ensues for the most part with a special adaptive feedback compensation filter, known as an FIR (Finite Impulse Response) filter. The frequency-limited amplified output signal, that is converted by the FIR filter into the compensation signal, serves as the input signal to the FIR filter.
The effect (characteristics) of the FIR filter, representative of most feedback compensation filters, is adapted with an adaptation unit that adjusts the filter coefficients. The adaptation is based on a comparison of an error signal, usually the input signal, with the amplified output signal. An important requirement for a successful adaptation is that both signals have experienced a substantially identical filtering before the comparison.
For the assembly of a feedback compensator, nodes and computer operations in the signal path are necessary that occupy space and require computer capacity. Furthermore, buffer storage (memory) is required in order to process the signal, for example by means of the adaptation unit and the feedback compensation filter. Such storage requires space on the hearing aid device chip and additionally must be supplied with power by the hearing aid device battery.
Various feedback compensators are known, such as from PCT Application WO 00/19605. The bandwidth of the compensation signal is thereby limited so that disruptions generated by the feedback compensation filter on the amplified output signal are minimized and limited to the unstable frequency range. The feedback compensator specified in PCT Application WO 00/19605 is designed to operate the memory efficiently; however, it has complicated signal paths or feedback paths with a number of elaborate computer addition operations and nodes.
For example, in an embodiment disclosed in PCT Application WO 00/19605 an input signal is split into three signals. The first two are directly merged back after each is filtered, and form the input signal for the hearing aid device signal processor. A compensation signal of an adjustable FIR filter is subtracted from this input signal before the processing for feedback compensation. The third signal, after filtering thereof serves as an error signal for an adaptation unit of the feedback compensation filter. In order to be able to successfully implement the adaptation, the compensation signal generated by the FIR filter is subtracted not only from the input signal for the hearing aid device signal processor, but also is additionally subtracted from the error signal before the error signal is supplied to the adaptation unit. This embodiment has three addition nodes, three splitting nodes, and three filters for the compensation of the input signal, and thus is complicated to build.
Another technique is based on the use of two hearing aid device signal processors. For this, an input signal is split into two signals. The first signal is supplied after filtering to a first hearing aid device signal processor. The second signal is filtered complementary to the first signal before it is merged with a compensation signal. It is then supplied as an error signal to an adaptation unit of an adjustable FIR filter, as well as to a second hearing aid device signal processor. The output signal of the second hearing aid device signal processor is filtered at the FIR filter and supplied to the adaptation unit, as well as being merged with an output signal of the first hearing aid device signal processor. In this technique, the selection of the complementary filter affects the hearing aid device signal processors.